1. Field of the Invention
This invention relates generally to electrical resistance heaters, and more particularly to heaters of the type incorporating PTC ceramic material which renders the heaters self-regulating at various preselected elevated temperatures.
2. DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR .sctn..sctn.1.97-1.99
Various types of resistance heaters are disclosed in U.S. Pat. Nos. 3,700,857; 4,486,651; 4,541,898; 4,544,828; 4,613,455 and 4,633,064.
U.S. Pat. No. '857 describes a heater consisting of a sintered mass of insulating refractory particles each of which has a thin film of electrically conductive material. U.S. Pat. No. '651 shows a ceramic heater having an element formed by sintering a mixture of molybdenum disilicide and silicon nitride.
U.S. Pat. No. '898 discloses a heating element composed of multiple finely divided particles of substance having a negative temperature coefficient of resistance, between which there are disposed areas of high resistance or areas of electrically non-conductive material. The resultant heater has different impedance characteristics according to the frequency of the A. C. wave that is applied to it.
U.S. Pat. No. '455 involves mixtures of different types of ceramics, namely silicon nitride as an insulating ceramic and a mixture of titanium carbide and titanium nitride as a conductive ceramic. The ceramics are mixed in powder form, and thereafter sintered. U.S. Pat. No. '064 illustrates a sintered heater element formed from an insulating ceramic powder such as silicon nitride, and a conductive ceramic powder such as MoSi.sub.2, WSi.sub.2, TiB.sub.2 or TiC.
Finally U.S. Pat. No. '828 relates to a heater formed by crushing PTC ceramic material, and mixing it with an insulating organic binder, to form the desired PTC resistor.
As presently understood, the devices disclosed in the above identified patents can be difficult to produce, since the ultimate resistance/temperature characteristics are largely determined by the relative proportions of the conductive material and insulating material, as well as the degree of sintering and the sintering temperature. It is believed that a reasonably close control of this resistance/temperature characteristic has, up to the present, been difficult to predict and achieve, as was uniformity of performance between different units of the same run.
In addition, where high temperatures on the order of 3000.degree. F. are being generated during operation of the heaters, problems occur with establishing satisfactory electrical contact to the elements. The use of metal electrodes can be prohibitive, since the melting temperature of metal contacts is well below the 3000.degree. F. figure noted above. As a result, most heaters of the prior art devices are intended to operate at temperatures well below this point.